1. Field of the Invention
The present invention relates to high-strength forged parts having a high reduction of area and a method for producing the same. More particularly, the present invention is concerned with high-strength forged parts superior in elongation and also in the balance of strength and reduction of area in a high strength region of about 600 MPa or more, as well as a method for producing the same. As typical examples of the “high-strength forged parts” according to the present invention there are mentioned near net shape forged parts, in which are included not only primary forged parts, but also precision-forged parts such as secondary and tertiary forged parts obtained by further forging (e.g., cold and warm forging) of the primary forged parts, and final products obtained by forming those forged parts into complicated shapes.
2. Description of the Prior Art
The use of forged parts is increasing in such industrial fields as automobiles, machinery, and electrical machines and appliances. Forged parts are generally produced by performing various forgings (workings) different in heating temperature and by subsequent refining (heat treatment) such as quenching and tempering. For example, in automobiles, hot-forged parts (heating temperature: 1100° to 1300° C.) and warm-forged parts (heating temperature 600° to 800° C.) are widely used for crank shafts, connecting rods and transmission gears, and cold-forged parts (heating at room temperature) are widely used for gears, pinion gears, steering shafts and valve lifters.
The forged parts in question are required to possess not only a high strength but also a high reduction of area. Such a requirement has been increasing recently. In this connection, the use of TRIP steel for such forged parts is now under study.
When retained austenite (γR) is produced in structure, the γR undergoes transformation (transformation-induced plasticity: TRIP) during deformation in working, with consequent improvement of ductility. TRIP steel utilizes this property effectively. Since TRIP steel is superior in both strength and ductility, it is widely used particularly for collision members and suspension members in automobiles. For example, in U.S. Pat. No. 5,505,796 there is disclosed a TRIP type composite phase steel (PF steel) comprising polygonal ferrite, bainite, and retained austenite. It is described therein that the PF steel possesses excellent punch stretch formability (ductility) and deep drawability and is superior in shock absorbability. In European Patent Publication 1,365,037, there are disclosed TRIP type composite phase steels each using tempered martensite or tempered bainite as a base phase structure and retained austenite as a second phase structure. It is described therein that these steel sheets are superior in all of strength, elongation, and stretch flange formability.
However, it turned out for the first time from the results of studies made by the present inventors that if the above TRIP steels are forged as they are by the foregoing method (quenching and tempering after forging), a coarse γR is produced in a large amount and acts as a starting point of fracture, with consequent occurrence of drawbacks such as cracking. Such drawbacks have heretofore occurred also in case of using other steels than TRIP steel, but in the studies made by the present inventors there occurred a marked lowering in the reduction of area and a marked deterioration of toughness.
Further, in the conventional method, two heat treatment steps, which are forging treatment and subsequent refining treatment involving quenching and tempering, are carried out separately, thus giving rise to the problem of an increase of cost and a lowering of both productivity and production efficiency.
Therefore, it is keenly desired to provide a novel high-strength forged part superior in elongation and also in the balance of strength and reduction of area even if the working ratio is set high, as well as a forging method which can produce such a forged part by a single heat treatment without going through such two heat treatment steps as in the prior art.